The United States has begun reconnaissance flights over Syria to track Islamic State jihadists but insisted it has "no plans" to coordinate with Syria on targeting the militants.
Numerous sources said foreign drones had been seen, with the Syrian Observatory for Human Rights reporting that "non-Syrian spy planes" had on Monday carried out surveillance of IS positions in the eastern province of Deir Ezzor.
The surveillance is seen as a precursor to possible US air strikes on jihadist positions, similar to those being carried out in neighbouring Iraq.
It comes after Syrian President Bashar al-Assad's regime said on Monday it was willing to work with the international community, including Washington, to tackle extremist fighters.
But American officials said they did not plan to coordinate with Damascus on targeting IS militants in Syria, despite Syrian insistence that any military action on its soil must be discussed in advance.
"There are no plans to coordinate with the Assad regime as we consider this terror threat," White House spokesman Josh Earnest said in Washington on Tuesday.
-- US militant killed in Syria --
US officials meanwhile said an American fighting for IS was killed over the weekend in Syria, underlining growing concerns about Westerners signing up for extremist groups in the Middle East.
The United States had been aware that Douglas McCain, 33, a one-time aspiring rapper and basketball fan from California, was in war-torn Syria, the White House said, confirming his death.
McCain, who converted from Christianity to Islam about a decade ago, was killed in fighting against the Al-Nusra Front, an Al-Qaeda-linked group blacklisted by the United States, US media reports said.
International concern about IS has been rising after a lightning offensive by the group through parts of Iraq and a string of brutal abuses, including the murder of US journalist James Foley.
On Monday, Damascus said for the first time that it was willing to work with the international community, including the United States and Britain, to tackle IS and Al-Qaeda's Syrian affiliate Al-Nusra Front.
But Foreign Minister Walid Muallem also made it clear that Syria would not accept unilateral military strikes by the United States or any other country.
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"Any violation of Syria's sovereignty would be an act of aggression," he said.
There have been few signs that the international community is willing to work publicly with Assad's regime, which has been engaged in a brutal effort to put down an uprising that began in March 2011.
However, sources in Damascus told AFP on Tuesday that the US was sharing intelligence garnered through its reconnaissance with Damascus through Iraqi and Russian channels.
"The cooperation has already begun and the United States is giving Damascus information via Baghdad and Moscow," one source close to the issue said on condition of anonymity.
- Syria planes hit jihadists -
The US began air raids against IS in neighbouring Iraq on August 8, in a bid to roll back its advances. It said that two air strikes near Irbil on Tuesday destroyed two IS armed vehicles and damaged another.
But US Chairman of the Joint Chiefs of Staff General Martin Dempsey has acknowledged that the group cannot be defeated "without addressing that part of the organisation that resides in Syria".
The White House says no decision has been taken on whether to carry out air strikes in Syria, although US aircraft have already entered Syrian airspace covertly at least once, on a failed mission to rescue hostages including Foley.
On Sunday, IS cemented its control over Raqa province, seizing the Tabqa military airport -- the last outpost controlled by the Syrian military -- in a battle that killed hundreds.
The group has also advanced in recent days in northern Aleppo province and controls territory in the oil-rich eastern province of Deir Ezzor.
Syrian war planes launched at least 12 raids using precision rockets against IS positions in Deir Ezzor on Tuesday, according to the Britain-based Syrian Observatory for Human Rights NGO.
In Iraq, the group has seen its momentum curbed in some areas by Kurdish forces backed by American air strikes, but it still holds significant areas that federal troops are struggling to regain.
On Tuesday, Iraq's air force carried out strikes against IS jihadists who have for months besieged the town of Amerli, where residents are short on food and water and face a "possible massacre".
"The people are still besieged and stranded there," said Eliana Nabaa, spokeswoman for the UN mission in Iraq.
There is "no possibility of evacuating them so far", and only limited humanitarian assistance is reaching the town, she said.